Aqueous solutions of chlorine dioxide, because of the high oxidation power of chlorine dioxide, are frequently used in water-treatment methods. The area of application ranges from the disinfection of drinking water and bathing water to treatment of service water and wastewater. In comparison with the classic oxidizing disinfectants chlorine and hypochlorite, chlorine dioxide is distinguished in use by an outstanding ecobalance. Thus, when chlorine dioxide is used, only small amounts of AOX (xe2x80x9cadsorbable organic halogen compoundsxe2x80x9d; overall parameter for all chlorine, bromine and iodine compounds which can be adsorbed to activated carbon and have very different hazard potentials) and virtually no trihalomethane (THM, haloforms) are formed.
It is known, for example from DE-C 843 999, for the preparation of chlorine dioxide, to proceed from a chlorite, for example sodium chlorite and to oxidize this to chlorine dioxide with an oxidizing agent, for example sodium peroxodisulphate, in aqueous solution according to the summation equation
2NaClO2+Na2S2O8xe2x86x922ClO2+2Na2SO4. 
To obtain a reaction rate as high as possible, the reaction solution in this known process is set to a pH of 5-9, if appropriate using a buffer, contains the oxidizing agent in a stoichiometric excess and can be heated to up to 65xc2x0 C. for further acceleration of the reaction. The chlorine dioxide forming is continuously expelled from the reaction solution by passing in inert gas and is collected in an absorption tower.
Although this known process gives a chlorine dioxide of very high purity at good yields, based on the chlorite used, it is highly restricted in its applicability and is little suited to industrial application xe2x80x9con sitexe2x80x9d, not only because of the high cost of equipment for separating the chlorine dioxide, but also because of the high explosion hazard of the gaseous chlorine dioxide. Furthermore, for many applications, for example for drinking-water treatment, there is also no possibility of avoiding the separation of the chlorine dioxide and using the reacted reaction solution as such for the disinfection, since this solution still contains too much residual chlorite and, moreover, is contaminated by toxic chlorate formed as by-product.
The object of the invention is to develop the known process in the manner of a xe2x80x9cone potxe2x80x9d reaction in such a manner that the reacted chlorine-dioxide-containing reaction solution can be used directly as such for the disinfection and is directly suitable for drinking-water treatment, for example. This object is achieved according to the invention by means of the fact that an aqueous reaction solution is prepared from chlorite and halogen-free oxidizing agent, which reaction solution contains the oxidizing agent at up to twice the stoichiometrically required amount and is set to a pH between 5.5 and 9.5 and is reacted at room temperature until the chlorite is at least virtually completely converted to chlorine dioxide.
The invention is based on the finding that it is possible, by means of systematic reaction procedure, to react the reaction solution to give a chlorine-dioxide-containing product solution which virtually no longer contains residual chlorite (in the ideal case no chlorite at all) and is also free of chlorate and other unwanted by-products. Surprisingly, it has been found that this is achieved by the interaction of a plurality of factors, namely
setting the pH of the solution to 5.5 to 9.5, preferably 6 to 9, as promptly as possible,
oxidizing agent excess in a molar ratio of chlorite to oxidizing agent between 1 and 2, preferably between 1.75 and 2,
sufficiently long reaction time at room temperature.